[SCEVValidator] Don't reorder multiplies in extractConstantFactor.

The existing code would add the operands in the wrong order, and eventually
crash because the SCEV expression doesn't exactly match the parameter SCEV
expression in SCEVAffinator::visit. (SCEV doesn't sort the operands to
getMulExpr in general.)

Differential Revision: https://reviews.llvm.org/D23592

llvm-svn: 279087